How do alternate WD Pasports?

What is the best way to altenate drives?  I keep one copy off site and swicth each month but I have to erase the file and have it fully redo the backup each time rather than incremental.  Is there a way to switch drives and just have it add the new files?

Hello,

WD Smartware does incremental backups. When ever you plug in a Passport drive to the computer the WD Smartware will only update the backup. To make sure this is functioning properly please install Smartware latest version ( Version 2.0.1).

When I connect the drive, it shows the file size and “state” when I removed it and says “backup completed”.  Do I need to restart smartware, or just wait and it will detect?  I have backup set to continuous.